Frequency modulation



July 8, 1941. o. E. now

7 FREQUENCY MODULATION Filed July 1, 1939 2 Sheets-Sheet 1 I MODULATOR Vbi. J 1 R R L ww a A" v F 90 A j 6 Z l M 41 M iv J, A I I A J C M ZDIRECT CURRENT v SOURCE s I BL INVENTOR. 0 VI E E. DOW

ATTORNEY.

FREQUENCY 42 MM TIPUERS c AMPLIFIERS, L-rc. Y 5

July 8, 1941. Q ow 2,248,045

' FREQUENCY MODULATION Filed July 1, 1939 2 Sheets- Sheet 2 Fig.5!

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MODULATION ,4, SOURC? (2L I F 55 1 r ,2 f o zzfsws R Fr, +4D/REC'TCURRENT ZZ saunas 4 Z3 T-\\/90 e =-6Z2 g l 1/ P INVENTOR.ORV/LL'E E. 00w BY ATTORNEY.

Patented July 8, 1941 FREQUENCY MODULATION Orville E. Dow, PortJefferson, N; Y., assign'or to Radio Corporation of America; acorporationof Delaware Application July 1, 1939, Serial NQ."282,406"

(Cl. Mil -171.5)

13 Claims;

This application concerns a new and improved circuit and means formodulating the frequency of wave energy of any frequency. The system ofthis invention isparticularly applicable to ultra-high frequency wavesignalling. By this system, an extremely wide band of signal potentialsmay be used and the frequency of wave energy may be deviated linearly inaccordance with the said wide band of signal potentials. The frequencyrange of the modulating potentials may be as used in telephony,television, facsimile; etc.

In its broadest aspect my invention involves an oscillation generator ofany desiredtype by means of which wave energy of the desired frequencyis produced and modulated in frequency. The frequency of operation ofthe generator is determined in part at least by reactance in thegenerator circuit. Where a tube generator is used the react-ance maycomprise the electrode leads within and without the tube envelope. Theextent of external connections forming the reactance depend in part atleast upon the frequency generated. In my invention one of thereactances comprises what has been termed in the art a tube reactance.This tube reactance comprises an electron discharge device having a pairof electrodes connected with the frequency determining circuit to form apart thereof with means for impressing generated oscillations on a pairI of electrodes in the tube in phase displaced relation to produce areactive effect between the said electrodes connected with the frequencydetermining circuit. By modulating theimpedance of the reactance tube atsignal frequency, the reactance in the frequency determining circuit isvaried correspondingly to thereby vary the frequency of the oscillationsgenerated.

A system somewhat similar to the improved modulator of the presentapplication has been disclosed in Crosby United States application#209,919, filed May 25, 1938.

In describing my invention reference will be made to the attacheddrawings wherein Fig. 1 is a wiring diagram including the principalelements of a frequency modulated oscillation gen erator arranged inaccordance with the present invention. Fig. 2 is a vector diagram usedin illustrating the relation of the several voltages, currents, etc,involved in the operation of the circuit of Fig. 1.- Fig. 3 is afundamental diagram equivalent to the circuit of Fig. 1. Fig. 3a is anadditional current and voltage vector diagram further illustrating-therelation of the var ious currents and Voltagesinthe oscillator cir-'cuit I of Fig. 1 forthe-prcduction of oscillations and also illustratesthemanner'in' which the reactive effect produced in circuit I I-an-dreflected in-cir-cuitl changes the frequency ofopera-. tion of thegenerator circuits. 1 Fig; e 'illustrates a modified formof" thefrequency modulator "of Fig. 1. In Fig. 4-the reactive eifectisinductivewhereas it is capacitive inFig. 1;;whi-leFig; 4c is a: vector diagramillustrating the operation of themodiiication of- Fig; 1. I

InFig; 1 numeral I is-an oscillator an'd II is amoduating means for the:osciliator. The 0s: cill-ator may. comprise an electron discharge -,deivice 4 havingits control grid 6 and anode 8 connected inreactivecircuits for the production of oscillations; The circuits may include aninductance. H) which is coupled to an inductance [2; The latterinductance maycbe ofthe output circuit having;; c-onnected thereto-*apair of output leads. One P int on-the inductance I*2-ris connected asshown to the anode Wot-a discharge device l6 having a pair of electronsystems in avsi-ngleenvelope. Separate electron dischargesyst-ems in"individual envelopes may be used "here;-- A point on the-inductance I2is also connected: by a ph-aseshifting condenser C to the control grid2010f one of the electron discharge systems. The. condenser C cooperateswith a phase shifting resistance R shunted" by an inductance L2,thepurpose of which will be given later. Another point on" theinductance I2 is simiiarly 'connected' to the anode 22 and grid 24 ofthe other electron 'd'isch-argesystem. A phase shitting? condenser C andresistance R and in du'ctanceLZ is'used here also.

Aswill be described more in detail hereinafter, these two tubesystemsform a symmetrical reactance infishunt to. the inductance [2 which iscoupled toatheinductance 10 so that the symmetricalfreactanwhas aneffect on the-oscillation .generatin'gand frequency modulation circuitsincludinginductance Ill; As will be pointed out morein detailhereinafter, when the im pedanc-esof the electron discharge systems ofthetube are modulated in phase by in-phase modulating potentials; thatis, varied in phase at signalnfrequency, the reactive efiectbetween theanodes and cathodes of the electron discharge systems is varied and thisreactance eifectbeing apart ofthe reactance of the-circuit includingreactance Ill'variesthe frequency of the oscillations generated atsignal frequency. Modulat ing potentials are supplied from any desiredsource, such as for example, as derived by scanning-asubject, by lead 30through resistances R and inductances L2 to the grids or controlelectrodes 20 and 24 in phase.

'As stated above, the frequency of operation of the oscillator is variedby changing the amplitude of the electron current flowing to the twoanodes of the tube I'B in accordance with the modulating voltage.

The radio-frequency voltages on the anodes M and 22 and control grids 20and 24 of tube l6 are in push-pull, i. e., the voltage on one anode is180 out of phase with respect to the voltage on the other anode, and thevoltages on the two grids are 180 out of phase also, but the voltages 1on the grid and anode of each system are displaced in phase bysubstantially 90. An advantage of the balanced circuit is that themid-point of the grid coils L2 does not need to be by-passed 1 for radiofrequency. This keeps the capacity I shunting the video modulator at aminimum.

A voltage is produced across the anodes of l6 j due to the circulatingcurrentin circuit I. The 1 voltage-applied to the control grids of 16 isad- 1 vancednearly 90 by the condensers 0 audio- 1 sistor R. Therefore,the pulses of current which irea-ch each anode will lead the voltage onthe said anode byabout 90 and-the reactance due I to the electroncurrent will be a capacitivereactance; pulses is controlled by themodulating voltage which is applied at 30' tothe control grids in Theamplitude of the said current parallel. Thus, if the controlgrid-sbecome-more positive due to the modulation voltage; the currentpulses increase in amplitude and the reactance due to the electroncurrent decreases since the voltages on the anodes of 16 remain aboutconstant in amplitude. This reactance is equal to the otherradio-frequency voltage from anode to ground divided by the fundamentalcomponent of the current pulses to each anode. This change in reactanceresults in a shift of the oscillator frequency.

The radio-frequency voltage on the control grids can be increased andshifted more nearly 90 by tuning out the capacity reactance from .gridto cathode which shunts the resistor R. In fact, if the impedance fromgrid to cathode is made slightly inductive, the phase shift can be madeto exactly equal 90. This. is accomplished by grid coils L2 shunting R.The voltage on the control grids may be made to lag the voltage on theirassociated anodes by reversing the positions of R and C or by crossingthe phase shifting circuits so they are connectedbetween anodes andgrids of opposite sides. This results in an inductive reactance in theoscillator circuit. A modification for producing this inductive reactiveeffect has been shown in Fig; 4.

The tube I6 may operate either class A or class C. If it operates classA, the control grids must have a remote cut-off characteristic so thatthe. alternating current component of electron current 'will vary inamplitude as the operating point is varied by the modulating voltage.The

radio-frequency exciting voltage on the control grids is constant inamplitude. If plate dissipation is the limiting factor, a greater degreeof a frequency modulation will be obtained if I6 -is f operated as aclass "C amplifier. The operation will be similar to an ordinary gridmodulated class Cfamplifier. Of course, frequency modulation can also beobtained by plate modulating 16,

f with the resulting advantage of less distortion and the disadvantageof greater modulating power required.

.The relation of 1 the voltagesand currents in Fig. l is indicated inthe vector diagram of Fig. 2. The notations used on the vector diagramcorrespond to those used on the schematic diagram. The peakradio-frequency voltage from anode to ground of one unit is representedby the vector 1 ep. All voltages and currents for the other unit will be180 out of phase with the corresponding vectors shown. il is theradio-frequency current which flows from the anode through the phaseshifting condenser C. After flowing through C the current il flows toground over three branches, i. e., control grid to ground capacity,resistance R, and inductance L2. These currents are represented by icy,z'R, and 21.2 respectively. eg is the grid to ground voltage and is inphase with 21?. and leads ep by 90 as is required. The emission currentpulses through the tube will be in phase with eg and hence lead ep by90". Thus the tube impedance due to its convection currents will be acapacitive reactance. 2L2 is larger than tag so that il lagszR slightly.The voltage drop across C is the difference between the vectors eg; andep and is represented by the dotted line connecting their arrows. Itmust lag il by 90 since C isa pure capacitive .reactance. Thus z'l mustlag z'R slightly. I

Fig. 3 is the equivalent circuit of Fig. 1. The condition foroscillation of circuit I is that e'g is .in phase with z'l asillustrated in Fig 3a. Thus if ZT, ,(the reactance due to the electroncurrent of I6) is varied, 12 will change and the factor 7'wMz'2 willthrow e'g out of phase with il. A change in the reactance of LI and CIand hence, in the frequency of operation is necessary to restore theresonant condition. .More in detail in Fig. 3 circuit I corresponds, tothe oscillator circuit I of Fig. 1. LI is the self inductance of thiscircuit, CI the circuit capacity and no the equivalent tube plateresistance and circuit resistance. e'g is the equivalent series voltagegenerated by the oscillator. As a necessary condition for oscillationsto take place e'g must be in phase with the circulating current il or I,i. e., the circuit I including the impedances reflected from circuitIImust be in resonance. L2 is the plate: to-plate inductance of circuitII, Fig. 1. C2 is the plate-to-ground capacity of the two elements oftube l6 in series. Z'T is the equivalent series impedance of themodulator tube l6 resulting from the convection currents. The generatedvoltage eg' is equal to the vector sum of the voltage drop il[r'p-l-yKXLL-XCD] due to the self impedance of I and the counter E. M.F. -jwMZ 2 in I due to the circulating current 22- in II. The voltage ofe2 is induced in II by the circulating current 12 of I. 22 is thecirculating current in II due to the induced voltage e2. 'XL2 is thereactance of L2 at the operating frequency, X02 the reactance of C2,etc.

As stated before an inductive reactance effect may be producedandmodulated at signal frequency to frequency modulate the oscillationsproduced in I. Such an" arrangement has been shown in Fig. 4. In thismodification the positions of R and C' are reversed and the inductanceL2 is placed in series with R and its reactance at I the operatingfrequencymade'equal to the reactance of C.- Ablocking condenser B isused in series withR to keep the plate voltage off of the controlgrid. Cmay then represent the grid-to-ground capacity of the tube. 'B is thedirectcurrent blocking condenser which has low impedance toradio-frequency currents; R, L2

and C compose'aseries circuit connected between the plate-and ground.The control grids are connected to this series circuit so as to includeonly C. The reactance of B2 is made equal to the reactance of C at theoperating frequency. The current il which flows in the series circuitwill be in phase with the plate-to-ground voltage ep as shown in vectordiagram of Fig. 4a. The voltage drop eL2 across L2 is equal to thevoltage drop eg across C but 180 out of phase. The voltage eg lags 1'1and ep by 90 and, therefore, the electron current pulses which reach theplate will lag the plate voltage by 90 and the reactance due to theelectron current will be inductive.

The inductances L3 have high impedance to radio-frequency currents, i.e., they act as chokes to the currents of the carrier frequency, andhave low impedance to the signal or modulation currents. L3 may beadjusted to act as series peaking coils for the modulating videofrequencies. C2 is a blocking condenser which has low impedance for thelowest frequency of the modulating voltage. Hg is a grid leak and E isthe bias for the control grids 20 and 24.

The output of the modulator may be supplied to any circuit. In practicethe frequency modulated output of the modulator is supplied to afrequency multiplier followed by a buffer stage and an additionalfrequency tripler. In a system arranged as disclosed here, the followingfrequencies were used and operation was found to be very satisfactorythroughout the range involved. The oscillator per so was operated at55.5 megacycles per second. This oscillator was frequency modulated inaccordance with television signals involving a band width of about 4megacycles. The modulation index was .111. The output of the oscillatoras modulated, was then tripled to a mean frequency of 166.6 megacyclesper second, the carrier now having a modulation index of .333. A bufferamplifier stage was excited by this energy and was followed by a triplerso that a carrier frequency of mean frequency of about 500 megacyclesper second was obtained with an index of modulation less than 1 coveringa total frequency spectrum of the order of 8 megacycles.

What is claimed is:

1. In a frequency modulation system an oscillation generator including afrequency determining circuit, and means for modulating the oscillationsgenerated including a pair of electron discharge tube systems eachhaving an anode, a cathode and a control grid, means coupling the anodesof said tube systems to points of substantially opposed radio-frequencyvoltages on said circuit, means for applying radio-frequency voltages tothe control electrodes of each of said electron discharge systems, thevoltages on the control electrodes and anodes of the respective systemsbeing displaced by substantially 90, and means for modulating theimpedances of the electron discharge systems in phase at signalfrequency.

2. In a frequency modulation system an oscillation generator including awave length determining circuit, and means for modulating the wavelength of the oscillations generated including a pair of electrondischarge tube system each having an anode, a cathode and a controlgrid, means coupling the anodes of said tube systems to points ofsubstantially opposed radio-frequency potential on said oscillationgenerator circuit, a condenser and resistance in series coupled to saidcircuit, a coupling between a point on said series connection and thecontrol grid of one of said electron systems, a second condenser andresisttime in series coupled to said circuit, a. coupling between apoint on said last series connection and the control grid of the othertube whereby the voltages on the anode and control grids of therespective electron systems are displaced by substantially and means formodulating the impedances of the electron discharge systems in phase atsignal frequency.

3. A system as recited in claim 2 wherein the capacitive reactancebetween the grid and cathode of each system is tuned out by aninductance to thereby obtain a more exact 90 relation between thevoltages on the anode and grid of each electron discharge system.

4. In a frequency modulation system an oscillation generator including afrequency determining circuit, and means for modulating the oscillationsgenerated including, a pair of electron discharge tube systems eachhaving an anode, a cathode and a controd grid, means coupling the anodesof said tube systems to points of substantially opposed radio-frequencypotential on said oscillation generator circuit, resistive means forapplying voltages from the anodes of said tube systems to the controlelectrodes of said electron discharge tube systems, capacitive; meanscoupling the control electrodes of said. tube systems to the cathodes ofsaid tube systems,. and means for modulating the impedances of the.-electron discharge tube systems in phase at signal;

frequency.

5. In a frequency modulation system an oscil-- lation generatorincluding an electron dischargedevice having electrodes connected inregenerative circuits for the production of oscillations, an inductancein said circuits, and means for modulating the frequency of theoscillations, including a pair of electron discharge tube systems eachhaving an anode, a cathode and a control grid,- means coupling theanodes of said discharge tube systems to points of substantially opposedhighfrequency potential on said inductance, a condenser connecting theanode of each of said tube systems to the control grid of each of saidtube systems, a resistance connected between the control grid andcathode of each of said tube systems, the values of said condensers andresistances being such as to produce a substantially phase quadraturerelation between the voltages on the anodes and control grids of therespective electron discharge tube systems, and a source of modulatingpotentials connected in phase between the control grids and cathodes ofthe tube systems.

6. In a frequency modulation system an oscillation generator includingan electron discharge device having electrodes connected in regenerativecircuits for the production of oscillations, an

inductance in said circuits, and means for modu-.

lating the frequency of the oscillations including a pair of electrondischarge tube systems each having an anode, a cathode and a controlgrid, a second inductance coupled to said first inductance, meansconnecting the anodes of said elec tron discharge tube systems to pointsof substantially opposed high frequency potential on said second namedinductance, a condenser connecting the anode of each of said tubesystems to the control grid of each of said tube systems, a resistanceconnected between the control grids and cathodes of each of said tubesystems, the values of said condensers and resistances being such as toproduce a substantially phase quadrature relation between the voltageson the anodes and control grids of the respective electron dischargetube systems, an. inductance in shunt to each of said resistances thepurpose of the said inductances being, to tune out the capacity betweenthegrid and cathode of each of said tube systems to thereby insure asubstantially phase quadrature relation between the voltages. on theanodes and control grids. of each tube system, and a source ofmodulating potentials connected inphase between the control grids andcathodes of the tube systems.

7. In a frequency modulation system an oscillation generator includingan electron discharge device having electrodes coupled in regenerativecircuits for the production of oscillations, a reactance insaidcircuits, and means for modulatingthe frequency of the oscillationsincluding a pair of electron discharge tube systems each having ananode, a cathode and a control grid, means'coupling the anodes of saidelectron discharge tube systems to points of substantially opposedhigh-frequency potentialon said reactance, a resistance coupling theanode of each of said tube systems to the control grid of each ofsaidtube systems, a condenser connected between the control grid andcathode of each of said tube systems the values of said condensers andresistances being such as to produce a substantially phase quadraturerelation between the voltages on the anodes and control grids of therespective electron discharge tube systems, and a source of modulatingpotentials connected in phase between the control grids and cathodes ofthe tube systems. V

Y 8..In a frequency 'modulation system an oscillation generatorincluding an electron discharge device having electrodes connected inregenerative circuits for the production of oscillations, a reactance insaid circuits, and means for modulating the frequency of theoscillations including a pair of electron discharge tube systemseachhaving an anode, a cathode and a control grid,

a second reactance coupled to said first reactance, means connecting theanodes of said discharge tube systems to points of substantially opposedhigh frequency potential on said second named reactance, a resistanceand inductance in series connecting the anode of each of said electrondischarge tube systems to. the control grid of eachof said electrondischarge tube systems,

a condenser coupled between the control grid and cathode of each of saidelectron discharge tube systems, the values of said condensers,inductances, and resistances being such as to produce a substantiallyphase quadrature relation between- ,the voltages on the anodes andcontrol grids of i anode of eachrof said tubes to the control grid ofeach of said tubes, a. condenser coupled between the controlgrid andcathode of each of said tubes, thevalues of said resistances;inductances and condensers being such as to produce. a substantiallyphase quadraturerelation between the voltages on the anode and control.grid of each of the tubes, a circuit: including; amindnctance. connectedin shunt to each of said condensers, and means for impressingcontrolling potentials on said last named inductances for controllingthe gain of said tubes to control the valueof the inductivereactance'between the anode and cathode of each of said tubes.

10. A variable reactance including, a pair of electron discharge tubeseach having an anode, a cathode and a control grid, means for applyingalternating voltages substantially in phase opposition to the anodes ofsaid tubes, a condenser connecting the anode of each of said tubes tothe control grid of each of said tubes, a resistance connected betweenthe control grid and cathode of each of said tubes, the values of saidcondensers and resistances being such as to produce a substantiallyphase quadrature relation between the voltages on the anode and controlgrid of each of said tubes, a circuit including an inductance connectedbetween the control grid and cathode of each of said tubes, the purposeof said inductances being to tune out the capacities between the gridand cathode of each of said tubes to thereby insure the phase quadraturerelation between the voltages on the anode and control grid of each ofsaid tubes, and means for controlling the gain of said tubes in phase tocontrolthe value of the reactances between the anode and cathode of eachof said tubes.

11. In a frequency modulation. system an oscillation generating circuitwherein oscillatory energy to be modulated flows, said circuit havingreactance, and means for modulating the reactance of said circuit andthereby modulating the frequency of the oscillations generated in'saidsystem including, a pair of similar electron discharge tube systems eachhaving a plurality of electrodes including a control electrode andcathode, means for impressing voltages from said oscillation generatingcircuit on a first two corresponding electrodes one in each of saidelectron discharge tube systems, other means for impressing voltagesfrom said generatingcircuit on a second two corresponding electrodes onein each of said electron discharge tube systems, the voltages impressedon the said first two corresponding electrodes and the voltagesimpressed on the second two corresponding electrodes beingsubstantiallyin phase quadrature, and'means for applying modulating potentials ofsubstantially like phase between the control electrode and cathode ofeach of said electron discharge tube systems.

12. In a frequency modulation system an oscillation generating circuitwherein oscillatory energy to be modulated flows, said circuit havingreactance, and means supplementing the reactance of said circuit with amodulated reactance to thereby modulate the frequency of theoscillations generated in said system including, a pair of similarelectron discharge tube systems each having a plurality of electrodesincluding a control electrode and cathode, means for impressingsubstantially phase opposed voltages from said oscillation generatingcircuiton a first two corresponding electrodes one in each of saidelectron discharge tube systems, other means for impressing voltagesfrom said generating circuit on a second two corresponding electrodesone in each of said electron discharge tube systems, the voltagesimpressed on the said first two corresponding electrodesfandthe voltageimpressed on the second two. corresponding electrodes beingsubstantially in phase quadrature, and means for modulatingthevpotential on the control electrodes of said tube systems relative to thecathodes of said tube systems in phase.

13. In a frequency modulation system an oscillation generating circuitwherein oscillatory energy to be modulated flows, said circuit havingreactance, and means for adding a modulated reactance to the reactanceof said circuit to modulate the frequency of the oscillations generatedin said system including, a pair of electron discharge tube systems eachhaving an anode, a control electrode and cathode, means for impressingvoltages of substantially opposed phase from said oscillation generatingcircuit on the anodes of said electron discharge tube systems, othermeans for impressing voltages from said generating circuit on thecontrol electrodes of said electron discharge tube systems, the voltagesimpressed on the anodes and control electrodes of each electrondischarge tube systems being substantially in phase quadrature, andmeans for modulating the potential between the control electrode andcathode of each of said electron discharge tube systems in phase atsignal frequency.

ORVILLE' E. DOW.

